This invention relates to negative temperature characteristic (NTC) thermistors for suppressing rush currents.
NTC thermistors are characterized as having lower resistance at elevated temperatures than at the normal temperature. Because of this unique characteristic, NTC thermistors are frequently utilized as a circuit element incorporated in a power source circuit of a device for suppressing the rush current which may flow into the source circuit at the instant when the power switch for the device is turned on.
As shown in FIG. 6, a prior art NTC thermistor 1 of a type enclosed inside a case and used for suppressing rush currents is generally structured so as to have elongated power-supply terminals 5 and 6 connected to electrodes 3 and 4 formed on two mutually opposite main surfaces of a circular disk-shaped thermistor element 2, both the thermistor element 2 and the power-supply terminals 5 and 6 being enclosed inside a heat-resistant resin case 7. The thermistor element 2 is supported by and sandwiched between the tips of the terminals 5 and 6 inside the hollow internal space of the resin case 7, while the other ends of the terminals 5 and 6 penetrate the body of the resin case 7, extending to its exterior.
One of the methods of improving the effect of such an NTC thermistor 1 to suppress a rush current has been to increase the volume of the NTC thermistor element 2 so as to increase its heat capacity such that the rise of its temperature due to the heat emitted by itself will be limited and the lowering of its resistance can be reduced. This method is not a practical one, however, because the cost of the NTC thermistor element takes up a large portion of the total cost of the product and the cost of the NTC thermistor element will increase if its volume, or its size, is increased.